Baffin Bay Earthquake – Canada – November 20, 1933

This earthquake had a magnitude of 7.4 but the outstanding point about it is that this part of Canada’s offshore in Baffin Bay is rarely hit with strong earthquakes.

On November 20, 1933, the largest instrumentally recorded earthquake to have occurred along the passive margin of North America occurred in Baffin Bay. Coincidentally, it also was the largest known earthquake north of the Arctic Circle. In spite of its size, the 1933 earthquake did not result in any damage because of its offshore location and the sparse population of the adjacent onshore regions.

The only known location that felt the earthquake was in Upernavik, Greenland. It was not felt in Thule to the north or in Disko Fjord to the south. One would have expected the earthquake to be felt in the closer northeastern coastal communities of Baffin Island but no reports were ever received.

The Baffin Island region continues to be active. In fact it is one of the most active regions in eastern Canada. Five magnitude 6 earthquakes have occurred here since 1933. The latest moderate-sized earthquake had a magnitude 4.8 and occurred on July 5, 2004. Analysis of seismograms of this earthquake shows strong evidence for strike-slip faulting, a condition that contrasts with the generally accepted belief that Baffin Bay is dominated by thrust faulting. The best-fitting solution consists of a large strikeslip sub event followed by two smaller oblique-thrust sub events.

All of these occur at a depth of about six miles. An instrumental magnitude of 7.4 was determined for this earthquake. Preliminary analysis of subsequent large earthquakes in Baffin Bay finds additional evidence for strikeslip faulting in the region. The results for Baffin Bay, together with those for other passive margin earthquakes, suggest strike-slip faulting may be more prevalent in these regions than was previously believed.

It was believed that Baffin Bay was formed by seafloor spreading between sixty and forty million years ago, but more recent evidence suggests that the seafloor spreading began much earlier, around sixty-nine million years ago. It has been difficult to define the ocean-continent boundary owing to the thick sediments in Baffin Bay. There is evidence for faulting in the basement rocks and older sediments in Baffin Bay and for slumping, which could be seismically related, in the younger sediments.

Although Baffin Bay is now known to be a very active seismic zone, considerably less was known about the 1933 event for a long time. Prior to the 1933 earthquake, the region was believed to be aseismic. Earthquakes of magnitude 6 and greater subsequent to1933 are noted in the International Seismological Summary and similar summaries, but it was only with the expansion of the Canadian seismograph network in the north during the 1950s and 1960s that these earthquakes could be put into any kind of regional context.

Estimates suggest that the earthquake catalogue for Baffin Bay has been complete above the magnitude 7.0 level since 1920, magnitude 5.5 since 1950, magnitude 4.0 since 1968 and is incomplete for magnitudes less than 4.0 for all time periods.

This contrasts sharply with the Charlevoix seismic zone in the long settled St. Lawrence Valley where the completeness years for the same magnitude levels are estimated to be 1660, 1900, and 1937, respectively, and where earthquakes of magnitude less than 0.0 can now be routinely located by a dense local seismograph network.

Historical seismic activity is not uniformly distributed throughout Baffin Bay but is concentrated in northwestern Baffin Bay on the Baffin Island side of the 6,000 feet bathymetric contour. To date no one has been able to correlate the seismicity with particular geological structures or geophysical anomalies. It has been suggested that it is related to the stresses associated with post-glacial rebound.